Resum:

In the recent decades, there has been a high rise in the development of renewable materials due to awareness in environmental care. Part of the research in such materials has focused on the development of biodegradable composites using natural fibres as reinforcement of 'green' plastics.
The use of biocomposites means a major reduction in the environmental impact of industrial components after their life cycle, thus, the development of tools for design with biocomposites could mean a breakthrough for its implementation in the industry.
In this thesis, the basis for the development of tools for modelling the mechanical behavior of biocomposites have been arise for first time through the development of a constitutive model based on the simulation of the mechanical behaviour through rheological elements. The seven model parameters have been adjusted by quasi-static tensile tests, while the model has been successfully validated by tensile tests at different strain rates. The constitutive model has proved to be valid for composites reinforced with cotton, flax and jute, allowing a greater understanding of the behaviour of those biocomposites, as the fact that the viscoplastic behaviour is mainly produced by fibres behaviour. The existence of a constitutive model for biocomposites opens the doors to its application in structural components of responsibility greatly reducing design costs.
Four different biocomposites were chosen to test the versatility of the constitutive model.
Biodegradable PLA composites were manufactured by compression moulding combining two types of PLA as matrix and three natural fibres (flax, cotton and jute) as reinforcement. Moreover, the ACC’s were manufactured by solving the surface cellulose of the fibres, which after a regeneration process forms the composite matrix. Thus, four different biocomposites were successfully manufactured in this thesis.
The parameters influencing the PLA based biocomposites manufacturing process (temperature, pressure, number of layers, type of fibre and matrix type) have been optimized to obtain a material with a strength greater than 100 MPa, indicating their potential application for replacing traditional composites, especially glass fibre composites. This could mean a large increase in the use of biocomposites in industrial applications such as automotive or aviation. However, it has been observed that biocomposites presents a viscoplastic behaviour with permanent deformations, which is far from the linear elastic behaviour until failure of traditional composites.
This has motivated the development of computational tools for biocomposites to predict their behaviour under dynamic conditions such as impacts or machining.
Impact test in drop tower were conducted in flax/PLA biocomposites, revealing a high energy absorption, above the absorbed by carbon fibre composites in the range of energies analysed.
The main failure mode was fibre failure, while delaminations were not found. Due to this differences in failure modes, the normalized residual strength observed in biocomposites was higher than that reported in carbon fibre reinforced composites.
Two different Finite Element Models were developed. First, a linear elastic model was used to reproduce the impact behaviour of ACC plates. Second, a model considering the influence of strain rate on the plastic behaviour of biocomposites was implemented to reproduce the impact behaviour of flax/PLA biodegradable composites.
Finally, the different mechanisms of damage induced in drilling had been studied. For this, the damage induced under different cutting speeds, advances and drill geometries was analysed, noting that in this case delaminations were neither found as failure mode, revealing a good cohesion between fibre and matrix. Is also detachable the damage reduction with increasing drill feed rate, which is a novelty that can reduce the processing times of these materials in the industry. These machining tests are the basis for the application of a numerical model based on the constitutive model defined in this work.

Author gratefully acknowledge the support of Spanish Ministry of Economy under the project DPI2013-43994-R and the Carlos III of Madrid University for the financial support during the last three years.